The present invention relates generally to apparatus for use in the art of burning-in and testing circuit components prior to their distribution and use, and, more particularly, to a system for driving circuit components during testing. Still more particularly, the present invention is directed to a distributed transmission line network for delivering test signals from a driver to circuit components under test.
According to present practices, integrated circuit ("IC") packages are mass-produced and installed in electronic circuits within highly sophisticated, complex and costly equipment. As with many mass-produced products, IC packages are prone to failure, in many cases at the beginning of operation. The complexity of the equipment within which such packages are installed makes post-installation failures highly undesirable. For example, when equipment reaches the final inspection stage of production, before failures are detected, the high level skills required for testing and repair add a significant cost to production expenses. Even more significantly, when the product has been installed in the field and a service technician must make warranty repairs, the costs thereby incurred can have a significant effect on profitability. As a result, manufacturers of electronic equipment are demanding ever greater quality and dependability in commercial grade IC packages.
Quality and dependability are enhanced substantially by detection of those IC packages likely to fail in the first few hours of operation, prior to installation of the packages in electronic equipment. Virtually all IC packages manufactured today, before they are sold for use in electronic equipment, are tested or "burned-in" for a predetermined period of time to detect IC packages that are defective or otherwise liable to fail prematurely. The burn-in process includes (1) placing the IC packages in sockets arranged in arrays on printed circuit boards ("burn-in boards"); (2) placing the burn-in boards with the packages, or devices under test ("DUTs"), mounted thereon in a chamber whose environment, particularly temperature, is controllable; (3) applying direct current (dc) biases to each package on each board in such a manner as to forward and reverse bias as many of the package's junctions as possible, and/or actively clocking each package to its maximum rated conditions, such application of dc biases and clock signals being accomplished substantially simultaneously to each package; (4) removing the burn-in boards from the chamber after the IC packages have been subjected to the environmental condition of the chamber and the biases and clock signals for a designated period of time; and (5) removing the IC packages from the burn-in boards. The present invention focuses primarily on step (3), which may also include the application of test signals to the DUTs.
A conventional burn-in board includes a plurality of sockets arranged in rows and columns. Signal lines, or traces etched onto the printed circuit card comprising the burn-in board, connect each pin of each socket to a tab on the connector edge of the board. Typically, the corresponding pins from each socket on each burn-in board are connected in parallel.
A conventional burn-in system or burn-in and test system includes an environmental chamber having means for receiving a plurality of burn-in boards. The connector edge on each burn-in board is received within an edge connector, generally outside the burn-in chamber. The edge connector may be attached to a second printed circuit card, or driver board, or it may be attached to a printed circuit backplane assembly, which also has edge connectors for receiving driver boards, as shown and described in U.S. Pat. No. 4,374,317.
The driver boards generate digital signals for exercising and testing the DUTs on the burn-in boards within the burn-in chamber. The digital signals are propagated through the edge connector and along parallel transmission lines on the burn-in board to the target pin on each DUT. This use of a parallel network of parallel signal lines to connect DUTs to drivers causes certain problems.
Each DUT on a burn-in board is a capacitive load that must be charged by the corresponding drivers. Similarly, the parallel signal lines connecting the drivers to the DUTs include capacitance that must be charged by the drivers. Thus, the use of a network of parallel signal lines to drive the DUTs means that the drivers must have sufficient power capability to deliver the current necessary to charge the capacitive load represented by each DUT and each signal line. One problem, therefore, with prior art burn-in systems is the need for relatively powerful drivers. Therefore, it would be desirable to develop a suitable signal line network employing a low-power driver capable of driving DUTs at high frequencies (above 1 megahertz).
Perhaps the most efficient low power, high frequency transmission line network, at least in theory, includes a single transmission line for each driver, with DUTs connected in series along the transmission line in a so-called "daisy chain." Significant difficulties in implementing such a network, however, have prevented its use thus far in connection with a burn-in chamber.
Two of the more significant problems are improper timing of input signals caused by propagation delay and signal distortion due to reflection. See Ching-Wen Hsue, "Clock Signal Distribution Network for High Speed Testers," 1989 International Test Conference, paper 8.2, pages 199-207. Propagation delay arises from a number of factors, but is more frequently a concern as the length of signal lines increases. Thus, using long signal lines may mean that the digital signals propagated along those lines will not arrive simultaneously at each DUT.
Most DUTs have a plurality of input terminals necessitating a plurality of digital input signals. For example, a DUT with 32 pins may require input signals at numerous input terminals, such as the clock input, the data inputs, the address inputs the enable input, and the reset input, each of which is connected to a separate driver. To test such a DUT properly, the digital signals at each of these terminals must arrive in a properly coordinated sequence. If the signals do not arrive in a properly coordinated sequence at each input terminal (a condition referred to as "signal skew"), the DUT will not be exercised in accordance with the desired test pattern, and the results of the test will be meaningless. Furthermore, as the frequency of the digital signals increases, the effect of skew becomes proportionately greater. Thus, any new transmission network employing transmission lines to connect DUTs sequentially must account for the problem of signal skew as a consequence of propagation delays in the transmission lines.
Reflection occurs when the original test signal spawns reflected signals on the transmission line. Reflection of the test signal arises because of impedance discontinuities in the transmission line. A reflected test signal causes waveshape distortion of subsequent digital signals as they propagate along the transmission line.
Signal distortion may also be caused by cross-talk between transmission lines. Because the transmission lines are located within a close proximity of each other, capacitive and/or inductive coupling between adjacent signal lines may cause interference known as cross-talk.
Conventional wisdom in the burn-in industry teaches that so-called "daisy chain" distribution of signals from drivers to DUTs is not feasible. It would be advantageous to develop a suitable signal distribution network capable of employing low-power drivers operating at high frequencies.